Edmund J. Peeler

Disease will limit future food supply from the global crustacean fishery and aquaculture sectors

Seafood is a highly traded food commodity. Farmed and captured crustaceans contribute a significant proportion with annual production exceeding 10 M metric tonnes with first sale value of

Non-native aquatic animals introductions have driven disease emergence in Europe

40bn. The sector is dominated by farmed tropical marine shrimp, the fastest growing sector of the global aquaculture industry. It is significant in supporting rural livelihoods and alleviating poverty in producing nations within Asia and Latin America while forming an increasing contribution to aquatic food supply in more developed countries. Nations with marine borders often also support important marine fisheries for crustaceans that are regionally traded as live animals and commodity products. A general separation of net producing and net consuming nations for crustacean seafood has created a truly globalised food industry. Projections for increasing global demand for seafood in the face of level or declining fisheries requires continued expansion and intensification of aquaculture while ensuring best utilisation of captured stocks. Furthermore, continued pressure from consuming nations to ensure safe products for human consumption are being augmented by additional legislative requirements for animals (and their products) to be of low disease status. As a consequence, increasing emphasis is being placed on enforcement of regulations and better governance of the sector; currently this is a challenge in light of a fragmented industry and less stringent regulations associated with animal disease within producer nations. Current estimates predict that up to 40% of tropical shrimp production (>

The application of epidemiology in aquatic animal health -opportunities and challenges

3bn) is lost annually, mainly due to viral pathogens for which standard preventative measures (e.g. such as vaccination) are not feasible. In light of this problem, new approaches are urgently required to enhance yield by improving broodstock and larval sourcing, promoting best management practices by farmer outreach and supporting cutting-edge research that aims to harness the natural abilities of invertebrates to mitigate assault from pathogens (e.g. the use of RNA interference therapeutics). In terms of fisheries losses associated with disease, key issues are centred on mortality and quality degradation in the post-capture phase, largely due to poor grading and handling by fishers and the industry chain. Occurrence of disease in wild crustaceans is also widely reported, with some indications that climatic changes may be increasing susceptibility to important pathogens (e.g. the parasite Hematodinium). However, despite improvements in field and laboratory diagnostics, defining population-level effects of disease in these fisheries remains elusive. Coordination of disease specialists with fisheries scientists will be required to understand current and future impacts of existing and emergent diseases on wild stocks. Overall, the increasing demand for crustacean seafood in light of these issues signals a clear warning for the future sustainability of this global industry. The linking together of global experts in the culture, capture and trading of crustaceans with pathologists, epidemiologists, ecologists, therapeutics specialists and policy makers in the field of food security will allow these issues to be better identified and addressed.

Treatment of gyrodactylid infections in fish

In this paper it is argued, using examples of disease emergence in aquatic animals in Europe, that the introduction of non-native species drives disease emergence by both extending the geographic range of parasites and pathogens and facilitating host-switching. Enteric red mouth disease and infectious haematopoietic necrosis of salmonids have extended their geographic range from North America to Europe with the import of live fish (Pimephales promelas) and rainbow trout eggs, respectively. Host-switching results in disease emergence when previously unidentified commensal organisms or known pathogen switch to new naïve hosts. The most serious endemic diseases of wild aquatic animals in Europe in recent years can be traced to the introduction of non-native species. Across Europe dramatic populations declines have occurred in native crayfish (e.g. Astacus astacus), oysters (Ostrea edulis) and eels (Anguilla anguilla), all which can be attributed, in varying degrees, to diseases (crayfish plague, Bonamia ostreae and Anguillicoloides crassus, respectively) introduced with non-native species. The severe adverse effects at a population level can be attributed to the lack of immunity in the new hosts. The impact of parasites more recently introduced to Europe, Sphaerothecum destruens (the rosette agent), and Batrachochytrium dendrobatidis, have yet to be fully determined. Both are generalists, with wide host ranges, and may present serious threats to native species. Aquaculture is the key driver for the introduction of non-native species. Most farming systems allow pathogen exchange between farmed and wild populations which underpins host-switching. Subsequently movements of animals between farms may result in the spread of newly emerged diseases. The introduction of non-native aquatic animals drives disease emergence, thus the ex-ante assessment of these hazards is severely limited. Generic risk mitigation measures (e.g. movement of disinfected eggs in place of live animals) and improved methods for rapid detection of new diseases are vital.

Risk-based methods for fish and terrestrial animal disease surveillance.

Over recent years the growth in aquaculture, accompanied by the emergence of new and transboundary diseases, has stimulated epidemiological studies of aquatic animal diseases. Great potential exists for both observational and theoretical approaches to investigate the processes driving emergence but, to date, compared to terrestrial systems, relatively few studies exist in aquatic animals. Research using risk methods has assessed routes of introduction of aquatic animal pathogens to facilitate safe trade (e.g. import risk analyses) and support biosecurity. Epidemiological studies of risk factors for disease in aquaculture (most notably Atlantic salmon farming) have effectively supported control measures. Methods developed for terrestrial livestock diseases (e.g. risk-based surveillance) could improve the capacity of aquatic animal surveillance systems to detect disease incursions and emergence. The study of disease in wild populations presents many challenges and the judicious use of theoretical models offers some solutions. Models, parameterised from observational studies of host pathogen interactions, have been used to extrapolate estimates of impacts on the individual to the population level. These have proved effective in estimating the likely impact of parasite infections on wild salmonid populations in Switzerland and Canada (where the importance of farmed salmon as a reservoir of infection was investigated). A lack of data is often the key constraint in the application of new approaches to surveillance and modelling. The need for epidemiological approaches to protect aquatic animal health will inevitably increase in the face of the combined challenges of climate change, increasing anthropogenic pressures, limited water sources and the growth in aquaculture.

Emergence of cold water strawberry disease of rainbow trout Oncorynchus mykiss in England and Wales: outbreak investigations and transmission studies.

Since Norway experienced the devastating Gyrodactylus salaris (Monogenea) epidemics in Atlantic salmon Salmo salar, there has been heightened interest in how to treat gyrodactylosis in fish. Here we summarize chemical treatments previously used against gyrodactylids and discuss the main problems associated with these control measures including efficacy, host toxicity, human health concerns and application of treatments. Unfortunately, for these reasons and because of the different methodologies and different parasite and host species used in previous studies, it is difficult to recommend effective chemotherapeutic treatments. However, we suggest a method for manual removal of gyrodactylids from the host suitable for use in small-scale research facilities.

An evaluation of the relative risks of infectious salmon anaemia transmission associated with different salmon harvesting methods in Scotland

Over recent years there have been considerable methodological developments in the field of animal disease surveillance. The principles of risk analysis were conceptually applied to surveillance in order to further develop approaches and tools (scenario tree modelling) to design risk-based surveillance (RBS) programmes. In the terrestrial animal context, examples of risk-based surveillance have demonstrated the substantial potential for cost saving, and a similar benefit is expected also for aquatic animals. RBS approaches are currently largely absent for aquatic animal diseases. A major constraint in developing RBS designs in the aquatic context is the lack of published data to assist in the design of RBS: this applies to data on (i) the relative risk of farm sites becoming infected due to the presence or absence of a given risk factor; (ii) the sensitivity of diagnostic tests (specificity is often addressed by follow-up investigation and re-testing and therefore less of a concern); (iii) data on the variability of prevalence of infection for fish within a holding unit, between holding units and at farm level. Another constraint is that some of the most basic data for planning surveillance are missing, e.g. data on farm location and animal movements. In Europe, registration or authorisation of fish farms has only recently become a requirement under EU Directive 2006/88. Additionally, the definition of the epidemiological unit (at site or area level) in the context of aquaculture is a challenge due to the often high level of connectedness (mainly via water) of aquaculture facilities with the aquatic environment. This paper provides a review of the principles, methods and examples of RBS in terrestrial, farmed and wild animals. It discusses the special challenges associated with surveillance for aquatic animal diseases (e.g. accessibility of animals for inspection and sampling, complexity of rearing systems) and provides an overview of current developments relevant for the design of RBS for fish diseases. Suggestions are provided on how the current constraints to applying RBS to fish diseases can be overcome.

An investigation into the prevalence of Renibacterium salmoninarum in farmed rainbow trout, Oncorhynchus mykiss (Walbaum), and wild fish populations in selected river catchments in England and Wales between 1998 and 2000

Cold water strawberry disease (CWSD), or red mark syndrome (RMS), is a severe dermatitis affecting the rainbow trout Oncorynchus mykiss. The condition, which presents as multifocal, raised lesions on the flanks of affected fish, was first diagnosed in Scotland in 2003 and has since spread to England and Wales. Results of field investigations indicated the condition had an infectious aetiology, with outbreaks in England linked to movements of live fish from affected sites in Scotland. Transmission trials confirmed these results, with 11 of 149 and 106 of 159 naive rainbow trout displaying CWSD-characteristic lesions 104 to 106 d after being cohabited with CWSD-affected fish from 2 farms (Farm B from England and Farm C from Wales, respectively). The condition apparently has a long latency, with the first characteristic lesions in the previously naive fish not definitively observed until 65 d (650 day-degrees) post-contact with affected fish. Affected fish from both outbreak investigations and the infection trial were examined for the presence of viruses, oomycetes, parasites and bacteria using a combination of techniques and methodologies (including culture-independent cloning of PCR-amplified bacterial 16S rRNA genes from lesions), with no potentially causative infectious agent consistently identified. The majority of the cloned phylotypes from both lesion and negative control skin samples were assigned to Acidovorax-like beta-Proteobacteria and Methylobacterium-like alpha-Proteobacteria.

Limited prevalance of gaffkaemia (Aerococcus viridans var. homari) isolated from wild-caught European lobsters Homarus gammarus in England and Wales

In aquaculture, harvesting presents an unavoidable risk of disease transmission. The spread of the disease infectious salmon anaemia appears to have been associated with harvesting in both Scotland and Norway. An assessment is made of the relative risks of disease transmission associated with various different means of harvesting farmed salmon based on the assumption that best practice is followed for each harvest method. The assessment is qualitative, quantitative data being absent for most of the processes involved. Slaughter on the farm risks the spread of infection to adjacent farms, whereas, infection at a processing plant may be rebroadcast by well-boats. The risks associated with transporting live fish in cages and of storing live fish near centralised processing plants are discussed. Provided the vessel does not release contaminated water in the vicinity of salmon farms, transport of live fish directly to the processing plant for immediate slaughter by sea may be the safest means of collecting harvest.

An assessment of the variation in the prevalence of renal myxosporidiosis and hepatitis in wild brown trout, Salmo trutta L., within and between rivers in South-West England

A cross-sectional survey of Renibacterium salmoninarum infection in farmed rainbow trout (RBT) and wild fish populations was carried out in 10 farms and six river catchments, respectively, in England and Wales. The majority of the wild fish were sampled in 1998 and the farmed fish in 2000. Grayling, Thymallus thymallus, and brown trout, Salmo trutta, were the main wild species sampled. Two fish, one grayling and one salmon, Salmo salar, were R. salmoninarum culture-positive, compared with 40 confirmed polymerase chain reaction-positive wild fish. The highest prevalence of R. salmoninarum infection was found in grayling in rivers with RBT farms with a history of R. salmoninarum infection. One hundred and fifty fish were sampled from each RBT farm, but none of the fish was found to be R. salmoninarum-positive. Evidence was found, for the first time, for the presence of R. salmoninarum in an eel, Anguilla anguilla.